Select THE_GEOM rather than *

Fix an exception if the geometry column is not the first column in the
table passed to ST_ShortestPath.
Make sure there is at least one geometry field
Merge pull request #345 from agouge/fix-st-sp

Fix  ST_ShortestPath exception
Fix an exception if the geometry column is not the first column in the
table passed to ST_ShortestPath.